Many devices fabricated as micro-machined electromechanical systems (MEMS), both sensor and actuator devices, and methods for manufacturing the same are generally well-known. See, for example, U.S. patent application Ser. No. 09/963,142, METHOD OF TRIMMING MICRO-MACHINED ELECTROMECHANICAL SENSORS (MEMS) DEVICES, filed in the name of Paul W. Dwyer on Sep. 24, 2001, which is assigned to the assignee of the present application and the complete disclosure of which is incorporated herein by reference, that describes a MEMS acceleration sensor and method for manufacturing the same. In another example, U.S. Pat. No. 6,428,713, MEMS SENSOR STRUCTURE AND MICROFABRICATION PROCESS THEREFORE, issued to Christenson, et al. on Aug. 6, 2002, which is incorporated herein by reference, describes a capacitive acceleration sensor formed in a semiconductor layer as a MEMS device. Other known MEMS devices include, for example, micro-mechanical filters, pressure sensors, gyroscopes, resonators, actuators, and rate sensors, as described in U.S. Pat. No. 6,428,713.
Vibrating beam acceleration sensors formed in a silicon substrate as MEMS devices are also generally well-known and are more fully described in each of U.S. Pat. No. 5,334,901, entitled VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,456,110, entitled DUAL PENDULUM VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,456,111, entitled CAPACITIVE DRIVE VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,948,981, entitled VIBRATING BEAM ACCELEROMETER; U.S. Pat. No. 5,996,411, entitled VIBRATING BEAM ACCELEROMETER AND METHOD FOR MANUFACTURING THE SAME; and U.S. Pat. No. 6,119,520, entitled METHOD FOR MANUFACTURING A VIBRATING BEAM ACCELEROMETER, the complete disclosures of which are incorporated herein by reference. Such vibrating beam accelerometers have been fabricated from a body of semiconductor material, such as silicon, using MEMS techniques. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, entitled METHOD OF MAKING AN ELECTROSTATIC SILICON ACCELEROMETER, and U.S. Pat. No. 4,945,765, entitled SILICON MICRO-MACHINED ACCELEROMETER, the complete disclosures of which are incorporated herein by reference.
As is generally well-known, a typical MEMS device, whether a sensor or an actuator, has a size on the order of less than 10−3 meter, and may have feature sizes of 10−6 to 10−3 meter. Moving parts within a device are typically separated by microscopically narrow critical gap spacings, and as such are highly sensitive to particle contamination, such as dust and other microscopic debris. MEMS devices are also sensitive to contamination arising from corrosive environments; humidity and H2O in either the liquid or vapor phase, which may cause stiction problems in the finished device; and mechanical damage such as abrasion. MEMS devices are often required to operate at a particular pressure or in a vacuum; or in a particular liquid or gas such as, for example, dry nitrogen; and in different acceleration environments from high-impact gun barrel munitions to zero gravity deep space applications. Such application environments aggravate the device sensitivity to contamination.
The manufacture of MEMS devices includes many individual processes. Each of the individual processes may expose the device to a source of contamination. This sensitivity to particle contamination poses a challenge to the structural design and microfabrication processes associated with these small-scale, intricate and precise devices in view of the desire to have fabrication repeatability, fast throughput times, and high product yields from high-volume manufacturing. MEMS devices are typically encapsulated and sealed within a microshell, i.e., between cover plates. The microshell serves many purposes, including shielding the micro-mechanical parts of the MEMS device from damage and contamination.
Traditionally, MEMS devices utilize a wafer stack or “sandwich” design of two or three stacked semiconductor silicon wafers, with the sensor or actuator device mechanism wafer being positioned in the center between two outside cover wafers or “plates” in a three-wafer device. The cover plates are formed, for example, in respective silicon wafers. Alternatively, the cover plates are formed in respective Pyrex RTM glass wafers.
In a two-wafer device, a single cover plate is mounted on top of the mechanism wafer. The cover plates are bonded to the mechanism wafer in a three dimensional MEMS device. A frit glass seal or another conventional mechanism bonds the cover plates to the mechanism wafer along their common outer edges or peripheries. Other common bonding mechanisms include, for example, eutectic metal-to-metal bonding, silicon-to-silicon fusion bonding, electrostatic silicon-to-silicon dioxide bonding, and anodic bonding for silicon-to-glass bonds. The cover plate wafer or wafers act as mechanical stops for movable portions of the mechanism wafer, thereby protecting the mechanism device from forces that would otherwise exceed the device's mechanical limits.
Electrical connections to the sensitive portions of the mechanism wafer typically require one or more bond wires that pass through window apertures in one cover plate and connect to conductive paths formed on the surface of the mechanism wafer. These conductive paths and the corresponding windows in the cover plate have traditionally been located within the interiors of the respective mechanism and cover wafers, thus being interior of the seals that bond the cover plates to the mechanism wafer along their respective peripheral edges. These internal windows can allow particulate contamination or moisture to invade the interior of the MEMS device during handling, transportation, testing or wire bonding operations, which can result in premature failure.
FIGS. 1 and 2 are plan and cross-sectional side views, respectively, of a first conventional MEMS device 10 of the prior art having the conventional conductive paths for routing signals into and out of MEMS devices. In FIG. 1 the prior art MEMS device 10 is shown open, i.e., without its top cover and with the MEMS sensor or actuator device mechanism removed for clarity. The prior art MEMS device 10 includes a MEMS sensor or actuator device mechanism bonded to the inner surface 28 of a bottom cover plate 22 at a position indicated generally at 12. As illustrated in FIG. 2, the MEMS device mechanism is formed in an interior portion of a mechanism wafer 14, which is an epitaxial layer of semiconductor silicon.
As illustrated in FIG. 2, top and bottom cover plates 20, 22 are sized to cover at least the device mechanism 12 and a peripheral frame portion 24 of the epitaxial silicon mechanism wafer 14 from which the device mechanism 12 is suspended. One or more electrical conductors 26, usually gold traces, are formed on an inner surface 28 of the bottom cover plate 22 and arranged for being electrically interconnected to with the device mechanism 12 by means well-known in the art. The electrical conductors 26 extend outwardly across the inner surface 28 of the bottom cover plate 22 to different conventional metal wire bond pads 30 that are positioned on the surface 28 of the bottom cover plate 22 outside the area occupied by the device mechanism 12. The electrical conductors 26 thus provide remote electrical access to the device mechanism 12.
The top and bottom cover plates 20, 22 are bonded or otherwise adhered to respective top and bottom surfaces 16, 18 of the mechanism wafer 14. The top and bottom cover plates 20, 22 each have a respective substantially planar inner surface 32, 28 that is bonded to the respective top and bottom surfaces 16, 18 of the mechanism wafer 14 using an appropriate conventional bonding mechanism 34 that is provided in a pattern in between the top cover plate 20 and the top surface 16 of the epitaxial silicon mechanism wafer 14, and between the bottom cover plate 22 and the mechanism wafer bottom surface 18. The bonding mechanism 34 is, for example, an adhesive bonding agent in a pre-form of glass frit, a eutectic metal-to-metal bond, a silicon-to-glass anodic bond, or an electrostatic silicon-to-silicon dioxide bond, as appropriate. The pattern of the bonding mechanism 34 is external to and may completely surround the device mechanism 12 and the wire bond pads 30.
As illustrated in FIG. 2, the top cover plate 20 is sized to cover at least the device mechanism 12 and the wire bond pads 30. Of necessity, a quantity of pass-through window apertures 36 are formed in the top cover plate 20 in alignment with the wire bond pads 30. In practice, the MEMS device 10 is cut out after the cover plates 20, 22 have been installed, so that the three stacked wafers, i.e., the device mechanism wafer 14 and the cover plates 20, 22, are all the same size, and the epitaxial silicon mechanism wafer 14 is completely and exactly covered by the top cover plate 20 (in a two-wafer stack) and the bottom cover plate 22 (in a three-wafer stack). The pass-through window apertures 36 in the top cover plate 20 provide access for connecting electrical wires 38 to the bond pads 30 for routing signals into and out of the device mechanism 12.
The pass-through window apertures 36 in the top cover plate 20 of the prior art device 10 illustrated in FIGS. 1 and 2 are located within the interior of the seals provided by bonding mechanisms 34 that bond the cover plates 20, 22 to the mechanism wafer 14 along their respective peripheral edges. These internal apertures 36 can allow particulate contamination or moisture to invade the interior of the MEMS device 10 during handling, transportation, testing or wire bonding operations, which can result in premature failure.
FIGS. 3 and 4 are plan and cross-sectional side views, respectively, of a second conventional MEMS device 40 of the prior art solution to the contamination problems inherent in the device 10 of FIGS. 1 and 2. The prior art MEMS device 40 has the conventional gold trace conductive paths 26 extended to a quantity of the conventional metal wire bond pads 30 positioned outside the seal 34 of the top cover 20. In FIG. 3 the MEMS device 40 is shown open, i.e., without its top cover, and with the MEMS sensor or actuator device mechanism removed for clarity. The MEMS device 40 includes a MEMS sensor or actuator device mechanism that is formed in the interior portion of the epitaxial silicon mechanism wafer 14, suspended from the mechanism wafer peripheral frame portion 24 and bonded to the inner surface 28 of a bottom cover plate 22 at a position indicated generally at 12.
The gold traces of electrical conductors 26 are formed on the inner surface 28 of the bottom cover plate 22. The gold trace electrical conductors 26 are electrically interconnected to the device mechanism 12 and extend outwardly across the inner surface 28 of the bottom cover plate 22 to the metal wire bond pads 30 that are positioned on the bottom cover plate inner surface 28 remote from the device mechanism 12 and which thereby provide remote electrical access to the device mechanism 12.
As illustrated in FIG. 4, the top and bottom cover plates 20, 22 are bonded or otherwise adhered to respective top and bottom surfaces 16, 18 of the mechanism wafer 14. The cover plates 20, 22 are formed having respective surfaces 32, 28 that are bonded to the respective top and bottom surfaces 16, 18 of the mechanism wafer 14 using an appropriate conventional bonding technique. The bottom cover plate 22 is sized to cover at least the device mechanism 12 and the supporting peripheral frame portion 24. The top cover plate 20 is sized to cover at least the device mechanism 12 and the supporting peripheral frame portion 24 while exposing the wire bond pads 30 on the bottom cover 22. The pass-through window apertures 36 in the top cover plate 20 are aligned with the wire bond pads 30 on the bottom cover plate 22, and thereby provide access for connecting electrical wires 38.
The pattern of the bonding mechanism 34 includes a portion 34a that lies between the device mechanism 12 and the wire bond pads 30 and overlies a portion of the electrical conductors 26. The wire bond pads 30 thus lie outside the pattern of the bonding mechanism 34 surrounding the device mechanism 12. The window apertures 36 in the top cover plate 20 also lie outside the confines of the pattern of the bonding mechanism 34.
The bonding mechanism 34 is optionally conventional anodic bonding when the cover plates 20, 22 are formed in respective Pyrex RTM glass wafers which is a well-known glass with a thermal expansion coefficient well matched to that of silicon. Anodic bonding can also be performed using thin glass films deposited by sputtering on a silicon substrate. Anodic bonding, however, fails to seal between the bottom cover plate 22 and the gold of the electrical conductors 26. The electrical conductors 26 thus prevent the bonding mechanism 34 from forming a hermetic seal.
Also, as illustrated in FIG. 4, the gold traces of the electrical conductors 26 are typically partially submerged beneath the bottom cover plate inner surface 28 in shallow troughs 42 etched in the cover plate inner surface 28. The partially submerged gold traces 26 also extend above the cover plate inner surface 28 by a small amount which may be on the order of 500 to 1000 Angstroms. Although small, this irregularity in the bottom cover plate inner surface 28 detracts from the seal by holding the inner surface 32 of the top cover plate 20 away from the bottom surface 18 of the mechanism wafer 14 so that no seal is formed in the immediate vicinity of the gold traces 26.
An alternative solution is disclosed in co-pending U.S. patent application Ser. No. 10/226,518, HERMETICALLY SEALED SILICON MICRO-MACHINED ELECTROMECHANICAL SYSTEM (MEMS) DEVICE HAVING DIFFUSED CONDUCTORS, filed in the name of Stephen C. Smith on Aug. 22, 2002, which is assigned to the assignee of the present application and the complete disclosure of which is incorporated herein by reference, wherein a hermetically sealed sensor or actuator device mechanism is electrically interconnected by diffused conductive paths to a plurality of wire bond pads that are located external to the hermetic seal.